GP Ultimate Bipe .40 + Saito .91?
The Saito 91 is only an ounce or two heavier than most of the .46's on the market today. I have seen one in a GP Ultimate bipe and it flew the plane very well. The plane can handle this engine just fine (the Saito 91 is lighter than the Saito 65).
Cheers!
Jollidude